On mips64, all arguments must be sign-extended, even unsigned types.

R=mstarzinger@chromium.org
CC=​sreten.kovacevic@mips.com, ivica.bogosavljevic@mips.com

Change-Id: If5229d34e1da684928f54bbcf389bb8e472d7d61
Reviewed-on: https://chromium-review.googlesource.com/868651
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50689}